The present invention relates to a process for producing sodium persulfate, which is widely employed in industrial fields at the present time as a polymerization initiator for polyvinyl chloride and polyacrylonitrile and as a treatment agent for a printed wiring board.
As a general process for producing sodium persulfate, there is known a production process by the reaction of ammonium persulfate and sodium hydroxide in an aqueous solution. It is necessary in the above-mentioned process that in the first place ammonium persulfate as a starting raw material be produced by an electrolysis method, and the resultant ammonium persulfate be concentrated and separated by vacuum crystallization, centrifugal filtration or the like and then taken out as a crystal. At this time, the solution containing the crystal (usually referred to as "mother liquor") is mixed with the liquid produced at a cathode and is used as a starting raw material for an anolyte.
The ammonium persulfate thus obtained is re-dissolved in the next step, and is transferred to the step of reaction with sodium hydroxide. In the aforesaid reaction step, a solution containing sodium persulfate is produced, then is concentrated and separated by vacuum crystallization, centrifugal filtration or the like and is subsequently taken out as a crystal. As mentioned hereinbefore, the process for producing sodium persulfate by the reaction of ammonium persulfate and sodium hydroxide necessitates quite long production steps and a number of steps, and, moreover, lowers the yield of the objective sodium persulfate based on the ammonium persulfate, thereby making itself far from economically advantageous.
Under such circumstances, several attempts have been made to produce sodium persulfate by direct electrolysis without passing through ammonium persulfate. For instance, Japanese Patent Application Laid-Open No. 56395/1975 (Sho-50) describes a process for producing sodium persulfate by the use of sodium hydrogensulfate as a starting raw material, which process, however, is impractical because of an extremely low current efficiency in the electrolysis.
In addition, Japanese Patent Publication No. 31190/1980 (Sho-55) describes a process for producing sodium persulfate by means of electrolysis through the use of a neutral starting raw material for an anolyte in the presence of ammonium ions, which process, however can not be said to be economical because of a low current efficiency being about 70 to 80% in the electrolysis. Further, the above-mentioned process suffers such disadvantages that the ammonium ions being contained in the objective crystal increase the content of nitrogen components in the objective sodium persulfate, and that the process necessitates a minute and attentive cleaning step in order to satisfy the ordinary requirement for the quality of sodium persulfate as the finished product, namely a purity of at least 99% and the content of nitrogen components of at most 0.1%. In spite of a number of efforts and endeavors having heretofore been directed towards the improvement of the production process, it is the real situation that an economical process for producing sodium persulfate has not yet been developed.